For many years, non-contacting oscillation measurements have been carried out successfully on blade discs of turbines, for example stationary gas turbines, by means of optical or inductive sensors, which produce considerable signals when individual blades on the blade disc pass by the sensor. Oscillation measurements such as these are required for monitoring purposes and in order to ensure safe operation of turbines.
In order to protect the individual blades on large-diameter blade discs against frequent bending loads, blades with so-called integral covering plates are now increasingly being used, and these result in an essentially closed covering strip structure on the blade disc. However, this closed covering strip structure has no target marks for optical or inductive sensors, which target marks could provide unambiguous signals for reliable analysis of the oscillation behavior of the blade disc. Suitable target marks with a defined arrangement would therefore have to be applied to the closed covering strip structure, from whose signals the oscillation behavior could be determined.
A further exacerbating factor in the case of gas turbines is that measurements must also be carried out reliably even when the locally occurring temperatures are high. For this reason, inductive measurement methods and ferromagnetic target marks that are required for them can be used only to a restricted extent, because the Curie temperature is exceeded.
U.S. Pat. No. 5,761,956 discloses an optical measurement method for determination of oscillations on a rotating blade disc of a gas turbine, in which different infrared thermal radiation from applied ceramic target marks with respect to the base body of the blade disc is detected. This measurement method cannot be used, however, until after the gas turbine has been operated for a certain time, because of the high temperatures which are required for infrared thermal radiation. While the gas turbine is being started up, the oscillation behavior cannot be monitored, however, which is disadvantageous to the operational reliability of the gas turbine.